First-principles Study Of Nanostructure Stability And Mechanical Behavior Of Light Al Alloys

Aluminum(Al)alloys,due to their excellent properties,such as light weight,high strength,corrosion resistance,fatigue resistance,wear resistance,impact resistance,good formability,non-toxicity,non-magnetism and recyclability,have been widely used in aerospace,high-speed railway,automobile,construction,ship,automation,electronic equipment,military industry and 3D printing fields.With the rapid development of modern science and technology,the demands for high-performance Al alloys are getting higher and higher,and seeking for a new generation of high-strength and high-toughness Al alloys is the research direction of the material field all the time,as well,it is also the key for China's Al alloy materials to move from the low end to the high end.However,as the Al metal with the face-centred cubic(FCC)structure has the high staking-fault energy(SFE),the twins are difficultly produced during deformation,which limits the development of Al alloys towards the high strength and high toughness.Solid solution strengthening is one of the main ways to improve the elastoplastic mechanical properties of metal materials,and adding appropriate alloying elements is the effective way to enhance the mechanical properties of Al alloys by strengthening and toughening.By experiments,the process of doping alloying elements in Al alloys is complex,and the experimental conditions are difficult to control,as well as the expensive costs and long development cycles.With the development of modern computational materials science,it not only overcomes the disadvantages of complex experimental conditions and high research and development costs,but also reveals the phenomena and mechanisms that cannot be observed in the experiment.The theoretical researches carried out in this paper,including nanostructure stability and mechanical behavior,can provide important theoretical basis and valuable scientific guidance for the preparation and design of high-performance Al alloys.Based on density functional theory(DFT),the first-principles calculation method can accurately calculate and simulate various mechanical properties of materials,which is an effective tool for studying the properties of materials.In this paper,the first-principles method is used for studying the effects of alloying elements on the stabilization of nanostructures,surface characteristics,stacking fault and deformation twinning in Al alloys,as well as their physical mechanisms.The interaction energies between solute atoms and nanostructures,various surfaces,stacking faults and twins are severally calculated,and the segregation behaviors of solute atoms are further analyzed.Combining the distribution models of solute atoms in Al substrate,such as Uniform distribution(UFD),Fermi-Dirac distribution(FDD),Gaussian-Like distribution(GLD)and Boltzmann distribution(BZD),the impacts of solute concentrations and temperatures on the intrinsic stacking fault energy(ISFE)of long-period stacking-ordered(LPSO)structures,twin boundary energy(TBE),surface energy and twinnability are systematically investigated,and the important roles of alloying elements in Al alloys are revealed,and then a series of research conclusions and research findings are obtained.The main research work and research results of this paper are summarized as follows:(1)The theoretical model of 9R phase supercell is established,and the influences of solute atoms(Cu,Fe,Ga,Ge,Li,Mg,Sc,Si,Sn,Sr,Ti,Y,Zn)on the 9R phase stabilization in high-performance Al alloys are studied by first-principles calculations,and then the interaction energies between the solute atoms and 9R phase are also calculated.Based on UFD and FDD models,we discuss the effects of solute concentrations and finite temperatures on the increments of ISFE in the 9R phase structure.Results show that high-concentration solute atoms(Ga,Ge,Sc,Si,Sn,Sr,Y)can dramatically decrease the ISFE of 9R phase,and then promote 9R phase stabilization,in addition,the capacity of these solute atoms on promoting 9R phase stabilization can be ranked as follows: Sr>Y>Sn>Ge>Si>Sc>Ga,where the isosurface contours of charge density also indicate that Sr atom has the best effect on promoting 9R phase stabilization among them,but other solute atoms(Cu,Fe,Li,Mg,Ti,Zn)go against the 9R phase stabilization,and high temperature also inhibits its stabilization.(2)The theoretical models of four types of LPSO structures,including 9R,12 R,15R and 18 R phases,are constructed,and the first-principles calculation method is used to explore the existence and stabilization mechanisms of 12 R,15R and 18 R LPSO structures compared with 9R phase structure in Al alloy.Based on the GLD completely random distribution model,the effects of solute atoms(Cu,Fe,Ga,Ge,Li,Mg,Sc,Si,Sn,Sr,Ti,Y,Zn)on the stabilizations of these LPSO structures are systematically investigated,and the interaction energies between these solute atoms and LPSO structures are calculated.The results suggest that the segregation behaviors of solute atoms behave different characteristics in these LPSO structures,and solute atoms(Ga,Ge,Si,Sn,Sr,Y)tend to be segregated in the stacking fault planes of these structures.Compared with other phase structures,Sr atom behaves the most stable effect on 15 R phase structure.High solute concentrations make for the structural stabilizations of these LPSO structures,while high temperature inhibits their stabilizations.For the ability to promote the stabilizations of different structural phases,solute atoms exhibit different characteristics: In 9R and 12 R phases,Sr>Y>Sn>Sc;In 15 R and 18 R phases,Sr>Y>Sc>Sn.The relevant studies provide an important theoretical basis for developing high-performance Al alloys with LPSO structures.(3)The theoretical models of five types of multilayer nanotwins structures(4,6,8,10 and 12Lnt)are built.At finite temperature,the effects of solute atoms(Sr,Y,Sn,Ge,Sc,Si,Fe,Ti,Cu,Ga,Zn,Mg,Li)on the stabilizations of nanotwins structures are investigated in Al alloys.Nanotwin energies have been calculated as well as the interaction energies between the solute atoms and multilayer nanotwins,and the results indicate that 6Lnt nanotwin structure is more stable than the others in pure Al.Based on UFD and FDD models,the impacts of solute concentrations and finite temperatures on the TBEs have been studied,and it is found that the solute atoms(Sr,Y,Sn,Ge,Sc,Si)can greatly decrease the TBEs in both models,but other atoms(Fe,Ti,Cu,Ga,Zn,Mg,Li)can increase the TBEs in UFD model,and high temperature can also increase the TBEs,which indicates that high temperature will inhibit the stabilizations of nanotwins.Specifically,the solute atoms(Y,Sr)can also greatly decrease the TBEs despite T =900K,and then make for promoting the stabilizations of nanotwins.This work is beneficial to guide the experiments to obtain a large number of nanotwins in Al alloys,so as to improve the mechanical properties of Al alloys.(4)It is rather difficult to measure the surface energy of metallic materials at finite temperature by experiment,thus,surface characteristics of them cannot be obtained.For this reason,this work develops a first-principles method to estimate the effects of finite temperature on the surface energy of Al alloys.The surface energies of four planes,namely,(100),(110),(111)and(112)planes,have been calculated as a function of the finite temperature,as well as the interaction energies between the solute atoms(Cu,Mg,Li,Sn)and these surfaces.Based on the UFD model,the influences of solute atoms on the surface energies of different surfaces are investigated.The results suggest that the finite temperature has a little effect on the surface energies of four surfaces in pure Al,while Sn atom can drastically reduce the four surface energies at relatively high solute concentration,which obviously increases the probability of forming a new surface,inducing more microcracks.However,Cu atom can inhibit the microcrack nucleation of(100)and(111)surfaces by slightly increasing their surface energies.Meanwhile,microcracks in Al alloys prefer to nucleate along the(111)surface due to the lower surface energy.This work is helpful to explore the surface characteristics and the mechanisms of microcrack nucleation of Al alloys at finite temperature.(5)The theoretical models of interstitial atoms(C and H)doped in the center of tetrahedron and octahedron in Al solid solution are established,and the effects of C and H atoms on twinning plastic deformation of Al solid solution are studied,and then the relative stability of C and H atoms in the center of tetrahedron and octahedron are compared,as well as their influences on the GPFE curve of pure Al solid solution,meanwhile,the interaction energies between interstitial atoms in different atomic layers and generalized stacking faults are also calculated.Based on the three improved twinning criteria and UFD distribution model,the effects of interstitial atomic concentration on twinnability parameters are further analyzed.The results show that in Al solid solution,the atom occupying form of C is in the octahedral center,while the one of H is in the tetrahedral center.Both C and H atoms can effectively reduce the minimum energy barrier of dislocation nucleation,thus promoting dislocation nucleation,but the increase of stable SFE is likely to cause the dislocation motion to be blocked.Moreover,C atom is more likely to promote dislocation nucleation than H atom.In addition,the high concentrations of C and H atoms can inhibit the twinning deformation at the crack tip and grain boundary,and then reduce the plastic mechanical properties at the crack tip and grain boundary,which may lead to brittle propagation and even fracture of the micro crack at the crack tip and grain boundary,and C atom is more likely to cause brittle fracture at crack tip and grain boundary than H atom.However,C and H atoms have little effects on the twinning deformation inside the intrinsic grains of Al solid solution.(6)The theoretical models of rare-earth elements(RE=Ce,Dy,Er,Ho,La,Tb,Yb)doped in different atomic layers in Al supercell are built,and the impacts of RE elements on twinning plastic deformation of Al alloys are investigated,and then the influences of RE atoms doped in the first nearest neighbor layer on the GPFE curve of pure Al solid solution are further analyzed,meanwhile,the interaction energies between RE atoms in different atomic layers and generalized stacking faults are also calculated.Based on the three improved twinning criteria and UFD distribution model,the effects of RE atomic concentration on the twinnability parameters are further analyzed.The results indicate that RE elements can effectively reduce the minimum energy barrier of dislocation nucleation and motion,and increase dislocation nucleation rate as well as dislocation motion,and then enhance the plastic mechanical properties of Al alloy,moreover,both La and Ce atoms have significant effects on promoting dislocation nucleation and motion.At the same time,both Ce and La atoms can promote twinning deformation at crack tip,and Ce atom is more likely to promote twinning deformation than La atom,and then improves the plastic mechanical properties at crack tip.At the grain boundary,except for La atom,the other RE atoms all inhibit the twinning deformation to some extent,making the brittle fracture easily,but La atom of high concentration can promote twinning deformation,and then increase the plastic mechanical properties at grain boundary.In addition,RE atoms(Dy,Er,Ho,La,Tb,Yb)have little effects on the twinning deformation inside the intrinsic grain,however,when the concentrations of Ce atom are within the range of 5%?7.1%,it will greatly promote the twinning deformation inside the intrinsic grain,and the ability of twinning deformation will decrease as the concentration increases again.